In the microelectronics industry, ongoing demand exists for improved device performance and for decreased device sizes and decreased device feature sizes. Reduced feature sizes provide dual advantages of increasing device feature density and increasing device speed.
Reducing feature and device sizes requires finding new ways to improve steps of the multi-steps process of manufacturing microelectronic devices. In methods for preparing many types of microelectronic devices, a step of removing silicon nitride is common. A thin layer of silicon nitride (Si3N4), usually deposited by chemical vapor deposition from silane (SiH4) and ammonia (NH3), can be useful in a microelectronic device as a barrier for water and sodium. Also, patterned silicon nitride layers are used as a mask for spatially selective silicon oxide growth. After being applied, all or a portion of these silicon nitride materials may require removal, which is commonly performed by etching.
The removal of silicon nitride from a substrate by etching can preferably be performed in a manner that does not damage or disrupt other exposed or covered features of a microelectronic device. Often, a process of removing silicon nitride is performed in a manner that preferentially removes the silicon nitride relative to other materials that are also present at a surface of a microelectronic device substrate, such as silicon oxide. According to various commercial methods, silicon nitride is removed from a microelectronic device surface by a wet etching process that involves exposing the substrate surface to concentrated phosphoric acid (H3PO4) at an elevated temperature, e.g., in a bath having a temperature in a range from 150° C. to 180° C. Conventional wet etching techniques for selectively removing silicon nitride relative to silicon oxide have used aqueous phosphoric acid (H3PO4) solutions, typically about 85 weight percent phosphoric acid and 15 weight percent water. Using fresh hot phosphoric acid, a typical Si3N4:SiO2 selectivity can be about 40:1.